The present invention relates to a coated printing paper, and more specifically, to a process for preparing a coated printing paper having excellent printability and producible at low cost. This invention claims the priority of Japanese Application Nos. 9-249078 filed on Sep. 12, 1997 and 9-263943 filed on Sep. 29, 1997. The disclosures of these two applications are hereby incorporated by reference.
In recent years, a demand for printing papers has been growing in commercial printing fields targeted at advertisements such as hand bills, catalogues, pamphlets and direct mails. These commercial prints are low in price as merchandise. However, it is important for them to attain their intended object as advertising media. Thus, there has been a high demand for these printed products with a satisfactory finish at a low cost.
To satisfy such a demand for high quality coated papers, paper manufacturers have been attempting to increase productivity while maintaining high quality, thereby cutting down on costs. Thus, they are using less expensive materials and chemicals, and endeavoring to produce highly cost-competitive products by (1) speeding, (2) broadening, (3) online manufacturing (production from paper making through coating to surface finishing), and (4) multiple coating. This has recently tended to make on-machine blade coating a popular means of production. Since this method performs paper making, coating, and surface finishing in the same plant, an apparatus (sheet run) used is very long. Once paper break occurs during operation in this situation, high production loss results, or paper feeding may require time or labor. Thus, efforts have been made to raise the manufacturing efficiency by maximizing labor- and space-saving. With this background, the necessity for minimizing the length of a drying zone has resulted in a tendency that a coating solution containing a pigment and an adhesive is coated on a base paper, and then as strong drying as possible is performed. To increase the evaporation rate, this drying is carried out at a dry-bulb temperature of 140.degree. C. or higher and an air pressure of 80 mmAq or higher. Radical drying, however, leads to marked migration of the binder (starch, latex) in the coating solution, increasing the frequency of printing nonuniformities, namely, mottling (uneven gloss in the single-color solid-printed area) and trapping unevenness (printing nonuniformity during multiple printing). These printing nonuniformities basically result from the nonuniform absorbing quality that the coated layer has for a printing ink, or its nonuniform water absorbing properties for damping water. Ideally, it is necessary to apply a coating solution more uniformly onto a base paper with minimal texture unevenness, thereby forming a coated layer having a uniform void structure with little density unevenness (Nishioka et al.; Journal of the Technical Association of the Pulp and Paper Industry, 41(3), 1987).
To correct gloss unevenness and printing nonuniformities, we have conducted studies on methods for forming a coated layer having a more uniform void structure. For example, we have found in our prior applications (Japanese Unexamined Patent Publication Nos. 370298/92 and 361695/92) that printing nonuniformities can be resolved by a method of wetting and heating a base paper or a coated paper, and then soft calendering it at a high temperature. We have also found in Japanese Patent Application No. 77862/97 that printing nonuniformities can be resolved by a method using a latex having a specific particle diameter, and starch having a specific reducing end group as well as a specific molecular weight.
In recent years, reductions in paper costs, and the use of thinner and lighter papers have been trends. With thinner papers, correction of opacity and print through is becoming a serious challenge for paper manufacturers. Thus, many attempts have been made to use spindle-shaped or columnar precipitated calcium carbonate having a large specific surface area as a coating pigment. From this point of view, the inventors have also used precipitated calcium carbonate having a special shape (spindle-shaped or columnar) as a pigment for a coated paper, and found methods for correcting opacity, resolving print through, and improving brightness (Japanese Unexamined Patent Publication Nos. 73695/94 and 73698/94). However, these methods were both applied to gravure paper employed with a different printing system. Mottling and trapping unevenness, problems characteristic of offset printing, remained unsolved with such use.
The precipitated calcium carbonate having a specific shape in customary use was produced by the reaction between milk of lime and carbon dioxide (called the carbon dioxide process) that followed a conventional manufacturing method. Thus, this material was high in unit price similar to kaolin, and using such precipitated calcium carbonate instead of cheap heavy calcium carbonate inevitably led to a marked increase in the manufacturing cost. We have also found that the use of spindle-shaped precipitated calcium carbonate, obtained by the carbon dioxide process, for an offset printing paper resulted in slow drying of ink. Since slow ink drying leads to set-off, ink drying properties should be improved further.
With the speeding of blade coating as stated earlier, the frequency of occurrence of stalactite or a bleeding problem (deposition of an aggregate from the coating solution on the edge of the blade) is known to increase. The coating solution using precipitated calcium carbonate having a special shape such as a fusiform, acicular or columnar shape tends to increase in viscosity at a high shear rate. This solution may slow high-speed operation using the blade.